The present invention relates to a method for aiming an antenna. The invention also relates to an apparatus for performing the aiming.
It is known to use permanent and semipermanent radio links in order to facilitate a typically bi-directional radio connection. Radio links relay telephone and data traffic as well as radio and TV signals between stations. The radio link is realized by means of electromagnetic radiation, i.e. radio waves, and the frequency of radio waves in a radio link may range from the RF wave band to the microwave region. Links may be 40 to 50 km long, in which case the link masts are typically 40 to 60 meters tall. Link antennas attached to the masts may be parabolic antennas, for example. Typically, there is a line-of-sight path between the radio link antennas so that radio wave propagation is free.
A known wireless transmission system for telephone and data traffic is the cellular-based public land mobile network (PLMN), such as GSM network, which facilitates wireless communication between a mobile station (MS), such as a mobile phone, and fixed parts of the system while the user of the mobile station moves in the system""s operating area. The radio links between the fixed parts of the system are typically a few kilometers or even shorter, whereby the types and sizes of the antennas used may vary. They may be e.g. antennas attached to masts or walls of buildings that are aimed at their targets, say at an antenna attached to another mast. As a general rule, cell sizes are becoming smaller, which will partly result in a growing number of radio links and, hence, more antenna installations and modifications.
Antennas are used to transmit and receive radio waves, and the characteristics of antennas are similar in both transmission and reception. An antenna does not radiate in the same way in all directions but its characteristics may be represented by a radiation pattern which describes the correlation between e.g. the field strength radiated by the antenna and the direction. Indeed many antennas radiate strongly in one direction only, whereby the radiation pattern of such an antenna usually has one main lobe and, in addition to that, weaker side lobes. The radiation pattern is an important factor in antenna design, and radio link antennas are normally very directional, i.e. the main lobe is both vertically and horizontally narrow. It is then obvious that such an antenna has to be aimed at another, receiving, antenna carefully and with adequate precision. It is often advantageous that an antenna be highly directional so that it causes less disturbance to other antennas. The direction of the main lobe of an antenna is also dependent on the construction of the antenna, whereby in conjunction with the mechanical assembly of the antenna structure the position of the antenna may be chosen such that the antenna is aimed at its target, say another transmitting or receiving antenna. Additionally, the mounting elements of especially light antennas weighing a few kilograms include adjustment means for fine-tuning the orientation of the antenna.
In the prior art, the aiming of a radio link antenna, for example, has been carried out in such a manner that the field strength of the antenna is measured with a so-called AGC voltmeter. Naturally the measurement has to take place at the target, say at the other end of the radio link, at which the antenna is aimed, and advantageously at the location in which the other antenna is to be, or has already been, mounted. Aiming is carried out in such a manner that first the antenna is pointed to the rough direction of its target, using e.g. a compass, after which the antenna is mechanically turned using its fastening or adjustment means, such as adjustment screws. At the same time the field strength is measured with the voltmeter, and by mechanically turning the antenna one attempts to find the field strength maximum both vertically and horizontally. Thus the main lobe of the antenna is aimed at its target and the antenna can be locked in its position.
A considerable drawback in the method described above is that simultaneously with the aiming, the antenna has to be transmitting a signal in order to make possible the use of a voltmeter. This means that the power is switched on in the antenna, whereby a person or persons working with it must be very careful in order to avoid hazardous situations. However, with the voltmeter, the antenna can be easily aimed erroneously if, by accident, one measures the field strength of a strong side lobe at the target. This can happen particularly in a situation where the coarse direction of the antenna is incorrect or the installation team is unfamiliar with the properties of the antenna. Check-up measurements for the side lobes prolong the time it takes to complete the measurements and installation. Furthermore it should be noted that signal reflections from the surroundings affect the measured field strength, distorting measurement results and causing aiming errors.
Another considerable drawback is that in practice the aiming takes two installation teams, each placed in one end of the radio link and measuring and aiming the respective antennas. The installation teams may communicate so as to give aiming instructions to each other.
It is an object of the present invention to eliminate the aforementioned problems of the prior art and to introduce an entirely new method and apparatus to be used in the aiming of antennas. The invention is based on the idea that an optical sight is utilized in the aiming of antennas.
The invention makes the aiming of antennas considerably quicker and simpler than in the prior art. A special advantage of the invention is that during the aiming work the radio link antennas do not have their power switched on as the antennas are not aimed by measuring the field strength. Another special advantage of the invention is that the aiming can be carried out by a single installation team, even by a single person, if necessary.
A further special advantage of the invention is that the antenna need not be installed in its place for the aiming. This way the antenna can be delivered for subsequent installation. This has the special advantage that the aiming can be carried out already at the radio link building stage, at which point it would indeed be impossible to power up the antenna, or when the fastening means for the antenna is being installed. By fastening the sight in a predetermined position into e.g. a mounting flange mechanism designed for the antenna, said position being such that the sight can be aimed at the direction of the main lobe of the antenna, the sight can be aimed at the target by altering the position of the fastening means. Advantageously the position is altered using adjustment elements, such as adjustment screws, used for fine adjustment. At the same time the fastening means for the antenna is set in a position corresponding to that in which the antenna, when installed, is aimed at the target.
An advantage of the invention is that the aiming can be carried out reliably and quickly, for the aiming is not affected by side lobes, reflections, other antennas, etc. In particular the invention is applicable in short-range, line-of-sight (LOS) radio links where the distance between antennas is typically less than 500 m. The aiming accuracy required is typically about 0.5 to 1.0 degrees, so it is obvious that in longer ranges special attention must also be paid to the clearances of the fastening means and to the fastening of the sight.
Known optical sights typically have such constructions that the person using the sight sets himself on the same line with the target and sight, i.e. on the aim line, behind the sight. However, the distance between the antenna fastening means and the wall of a building, for instance, is typically very short, resulting in the problem that the person aiming the antenna does not have enough room to position himself behind the sight. In the apparatus according to the invention this problem is solved by fitting in the sight a prism or mirror which diverts the direction of the aim line. These can be fitted in a simple manner in front of or behind the sight, for example. Advantageously a prism is used which causes a smaller error in the refraction of light than a mirror. A considerable advantage of the invention is that the sight, the person or both can turn e.g. 90xc2x0 to the side where there is enough room for working. In addition, a red dot sight is parallax-free, so the point of aim will not change even if it is viewed slightly off the center axis. This property reduces the possibility of error in installation.
A special additional advantage of the invention is that it can utilize sights usually known to be used in conjunction with bows, firearms, air guns or color cartridge guns, for example. With the invention, such sights will be used in a novel fashion and in a new operating environment as aiming devices, which considerably adds to their versatility.